Unleaded MGs Uncovered

Part 1

This very specific hot potato has been baking so long now that it should be burnt to a crisp. However, judging from the large number of questions that this topic is continuing to raise it looks to have considerable ‘cooking time’ left. Since the questions asked are usually the same whichever model of MG is considered, the following general guidance should help to answer most questions.

I think that by now most owners will have taken on board that unleaded fuel is not as friendly to engine parts within the combustion area. Exhaust valves, seats and guides being the most sensitive. I think that most will also have gathered that leaded fuel will only have very limited supply EEC after the millennium. Even where available the volume will be strictly controlled, and additionally I suspect the same function will be achieved through punitive price control. Outside the EEC many areas have been unleaded for many years and still people enjoy their MGs so the prophets of doom will have to look elsewhere.

The specific problem for owners of pre 1990 MGs is that we often do not have clear and concise information on what is the best course of action for their particular car. Many manufacturers have been quite definite in their technical information and owners have a very clear idea of what to do. Although Rover or more precisely Austin Rover and also the various previous administrations, have issued information, that has been proved to be contradictory. Conflicting sources of technical information from within the same large organisation simply confuses the unfortunate MG Owner.

The only clear fact is that Rover will only publicly approve the use of unleaded for those engines specifically approved at the time of their production. Conversion of any engine will not gain approval, even if the specification is brought to exactly that of an approved unleaded specification. Owners should bear this policy of blanket refusal to retrospectively approve any conversions in mind and balance it against other facts.

I will hopefully try and clear some of the ‘fog’ and enable owners of post 1955 cars to make more rational decisions. I do not endorse any product or any particular company, but I am a firm believer in the engineered route, which is the route always chosen my the manufacturers, for its ability to not only match the long term reliability of engines running leaded but also to exceed that level.

When using unleaded fuel in place of leaded it creates a hotter and more degrading environment for the components in the combustion chamber and exhaust. The octane rating of unleaded is often associated as being lower but this is a separate factor and can be altered to match or exceed that of leaded. However since 95RON unleaded is going to be the UK choice to make on economic grounds I will cater for this in the recommendations.

Over the years that leaded fuel was available, engine designers and company ‘bean counters’ were able to specify the use of lower quality and cheaper materials, yet still achieve acceptable engine life. The use of unleaded demanded changes to higher quality materials to maintain the same reliability. In a perfect world the change from one specification to another would be clearly defined but of course this is far from the case. With different markets around the world changing to unleaded at different times, there is not always a clear change point in any given range of cars, as changes were often made for that specific market only, leaving others on the original specifications.

The opposite can also be true where it is deemed more cost effective to introduce changes for all markets at the same time, regardless as to whether they are needed. The classic example of this is with the adoption of the rubber bumpers on the MGB for North American requirements and all other markets had to have them as well. Unfortunately when looking at the changes for unleaded fuel such a clear change point isn’t there.

So much for the confusion let us get down to business…

The first thing to say is that many owners have reported that their non unleaded compatible engines have run for considerable time and many miles on a 100% diet of unleaded without any adverse effects. There is no arguing with those experiences, but the fact remains that if the same operating conditions were to be applied with the engine running on leaded fuel, the service life of that engine would be higher and if engineered to run unleaded, higher still. It is impossible to quantify how long any particular engine will run before problems manifest themselves. The manner and conditions of use will be the factors that dictate this time scale.

I think that all would agree that after say 75,000 miles most MGB engines benefit from some decoking and attention to valve sealing. I think that most would also agree that if the same engine had been heavily used, with full throttle and peak revs being used at every opportunity, it would probably need major work at 15,000 miles, accompanied by comments such as ‘What else do you expect!’

One common factor now is that very many classic MGs are vehicles for pleasure and not required for daily transport. This means that they will not be subject to the same pressure of use and usually this leads to a much easier life. This significantly eases the stresses and pressure within the cylinders and this is reflected in less component erosion. Consequently with many MGs only being used for limited annual mileage’s, it follows that a simple change to unleaded use may well see years of normal use before any problem surfaces.

Experiences from North America, where over the last 20 years unleaded has ruled the roost, indicate that many owners simply continue to use their unconverted cars as normal. They do any remedial work on their engines on an ‘as needed’ basis and have found that the time scales do not seem to have panned out that different to the time when leaded was available. Different markets will have different fuel blends, different driving conditions and different climates, so variations will be seen. These are unlikely to be hugely different, so the experiences of others can be used as a guide as to how our situation will pen out over the next few years.

Now let’s look at specific engine basics and see whether any manufacturer developed engineering is built in…

The ‘A’ series engine.
The in-line versions of the ‘A’ series, as fitted to the Midgets, were not engineered to give the required standard level of reliable service with the continual use of unleaded fuel, therefore the engine requires additional protection. The engineered approach is available from a number of specialists. Valve seat inserts and guides being the basic requirements, original spec valves being usually of high enough quality to be retained but be guided (no pun intended) by the engineering shop. The same period transverse applications of these engines, MG 1100 and MG 1300, follow exactly the same route as the in-line engines.

The MG Metro has a slightly more confused path. Very late MG Metros (those with the high filler neck) were equipped for unleaded and can use it without modifications. Earlier engines have to follow the same route with valve seat insets and guides, but the original valves are of a high enough quality to be retained. One option would be for leaded cars to use the complete head of an unleaded engine, there being no specification changes below head gasket line. Engine codes with (12H) E24, E25, E35, E39, E40, E41, E42, E48, E67, E68, E69, E70, E71, E72, E73, E74, E75, F01, F02, are all unleaded compatible, yet only the two latter F series are fitted to MGs.

A slight anomaly seems to stem from the Swiss and German 1986 on model year cars. My (personal) understanding is that these models were able to run unleaded and the parts used in the cylinder heads seems to support this, with the use of inserts and guides as found in the unleaded spec engines.

The MG Metro Turbo was never engineered to run unleaded but does benefit from the use of valves and guides that are high enough quality.Inserts that are listed are also the same units as fitted to unleaded engines so the fitting of these will enable the car to use unleaded. Those worried about the turbo should bear in mind that these turbo units are also used with fully unleaded engines and as such need no modifications.

The ‘A’ series being such a popular engine over many decades has spawned a huge tuning industry and this has led to the production of a number of alternative heads. Probably the best known were the eight port alloy conversions used on some works Minis along with 4 Amal carburettors. The purpose of these was solely to increase performance, now however, the re-emergence of these heads adds an additional dimension. I am led to believe that the current versions are all unleaded compatible.

The conversion factor doesn’t stop there as there are several other alternative heads made for the ‘A’ series. The best known are the Jack Knight and KAD twin cam 16 valve conversions. These are in fact complete engine conversions rather than alternative head options, and I mention them only to provide a full picture. Naturally these are not only unleaded compatible but also very efficient.

The ‘B’ series engine…
This engine follows the same route as the ‘A’ series in that in most applications it is not engineered to run unleaded fuel. For the UK this can be regarded as total non-compatibility, but the North American market dictated that from the early 1970s some cars were so engineered. The same specialists who cater for the ‘A’ series conversions also cater for the ‘B’ series and the same route is to be followed.

As far as the North American market is concerned, a look in the parts books and reading contemporary features seems to indicate that the factory followed two engineered routes to equip the B series in the MGB to cope with unleaded. One was the use of valve seat inserts and the other was to harden the cast iron of the head. The latter process being the only option used on post 1976 engines from the factory. Valve guides were the same throughout, but there were some variations with valves. Information in this area is a little patchy and fragmented.

There are many suppliers of heads engineered to run with unleaded that are proven with many years of satisfactory running. Many of these conversions are incorporated into a series of other power tuning modifications and with this comes considerable extra stress. Again these have been well proven over time, miles and the numbers converted. These conversions apply equally to a very late B series from an MGB to a 1489cc early B series found under the bonnet of an MGA or Magnette.

There are a several after market cylinder heads available for the B series; with the ‘Derrington Alloy Cross flow’ seven port probably being the best known. This design was created to provide a substantial improvement in performance over the original cast iron five port design and it did, although developments with the original cast iron heads can now match it for performance, but not in other areas. Speak to Brown & Gammons or Webcon UK for these. Other alloy heads in the original five port designs are also available from suppliers such as Moss. All alloy heads feature hardened seat inserts and should be capable of running unleaded, but double check with the supplier first.

Cost of these alloy heads is high, but bearing in mind that the number of serviceable cast iron units is falling rapidly, this provides another avenue of supply, potentially for years to come. One reason for the reducing number of cast iron heads is through fatigue and cracking, the process of conversion to run unleaded can in many cases reclaim some heads that would otherwise be scrap. This is a reason not to throw away your heads that have cracks between the valve seats, at least not until it has been examined by an engineering shop.

I mention the MGA Twin Cam last, as this will be the rarest ‘B’ series derivative engine. It has an alloy head and the principles that apply to other alloy heads apply equally here too. As there is a dedicated group of enthusiasts who have kept the Twin Cam going for over four decades after a pretty calamitous introductory period of failed engines, they are well qualified and versed to continue the model’s life for another four plus decades.

The ‘C’ series engine
The ‘C’ series as fitted to the MGC uses the same basic materials and has the same basic design and use limitations as the ‘A’ and ‘B’ series engines. As such the same engineering requirements are also needed.

This is a comparatively rare engine, but during its lifetime alternative alloy cylinder heads have been made. The all alloy engines as featured in the factory competition cars are the best known but I have seen another all alloy head of an unknown source. In either case rarity will preclude them from being anything other than a point of conversation.

.. continued